Because of their strength, heat resistance, and chemical resistance, polyester fibers and films are an integral component in numerous consumer products manufactured worldwide. Most commercial polyester used for polyester fibers and films is polyethylene terephthalate (PET) polyester. Because polyethylene terephthalate forms a lightweight and shatterproof product, another popular use for polyethylene terephthalate is as a resin for containers, especially beverage bottles.
Before 1965, the only feasible method of producing polyethylene terephthalate polyester was to use dimethyl terephthalate (DMT). In this technique, dimethyl terephthalate and ethylene glycol are reacted in a catalyzed ester interchange reaction to form bis(2-hydroxyethyl) terephthalate monomers, as well as a methanol byproduct that is continuously removed. These bis(2-hydroxyethyl)terephthalate monomers are then polymerized via polycondensation to produce polyethylene terephthalate polymers.
Purer forms of terephthalic acid (TA) are now increasingly available. Consequently, terephthalic acid has become an acceptable, if not preferred, alternative to dimethyl terephthalate as a starting material for the production of polyethylene terephthalate. In this alternative technique, terephthalic acid and ethylene glycol react in a generally uncatalyzed esterification reaction to yield low molecular weight monomers and oligomers, as well as a water byproduct that is continuously removed. As with the dimethyl terephthalate technique, the monomers and oligomers are subsequently polymerized by polycondensation to form polyethylene terephthalate polyester. The resulting polyethylene terephthalate polymer is substantially identical to the polyethylene terephthalate polymer resulting from dimethyl terephthalate, albeit with some end group differences.
Polyethylene terephthalate polyester may be produced in a batch process, where the product of the ester interchange or esterification reaction is formed in one vessel and then transferred to a second vessel for polymerization. Generally, the second vessel is agitated and the polymerization reaction is continued until the power used by the agitator reaches a level indicating that the polyester melt has achieved the desired intrinsic viscosity and, thus, the desired molecular weight. More commercially practicable, however, is to carry out the esterification or ester interchange reactions, and then the polymerization reaction as a continuous process. The continuous production of polyethylene terephthalate results in greater throughput, and so is more typical in large-scale manufacturing facilities.
When the polymerization process is complete, the resulting polymer melt is typically extruded and pelletized for convenient storage and transportation before being transformed into specific polyester articles (e.g., filament, films, or bottles). The latter kinds of steps are herein referred to as “polyester processing.”
In both batch and continuous processes, a high activity catalyst is often employed to increase the rate of polymerization, thereby increasing the throughput of the resulting polyethylene terephthalate polyester. The high activity catalysts that are used in the polymerization of polyethylene terephthalate polyester can be basic, acidic, or neutral, and are often metal catalysts.
Primarily, the traditional polymerization catalysts used in the formation of polyethylene terephthalate from both terephthalic acid and dimethyl terephthalate contain antimony, most commonly antimony trioxide (Sb2O3). Although increasing production rates, polymerization catalysts like antimony trioxide will eventually begin to catalyze or encourage the degradation of the polyethylene terephthalate polymer. Such polymer degradation results in the formation of acetaldehyde, the discoloration (e.g., yellowing) of the polyethylene terephthalate polyester, and reduction of polymer molecular weight.
Furthermore, the recent availability of “hotter” catalysts that can significantly increase throughput has generated a corresponding need for better stabilization of the resulting polyester. U.S. Pat. No. 5,008,230 for a Catalyst for Preparing High Clarity, Colorless Polyethylene Terephthalate is exemplary of such an improved catalyst. To reduce the degradation and discoloration of polyethylene terephthalate polyester, stabilizing compounds are used to sequester (“cool”) the catalyst, thereby reducing its effectiveness. The most commonly used stabilizers contain phosphorous, typically in the form of phosphates and phosphites. The phosphorous-containing stabilizers were first employed in batch processes to prevent degradation and discoloration of the polyethylene terephthalate polyester.
Although adding a stabilizer to the polymer melt in a batch reactor is a relatively simple process, numerous problems arise if the stabilizers are added in the continuous production of polyethylene terephthalate. For example, while early addition of the stabilizer prevents discoloration and degradation of the polyester, it also causes reduced production throughput (i.e., decreases polycondensation reaction rates). Moreover, such stabilizer is typically dissolved in ethylene glycol, the addition of which further slows the polymerization process. Consequently, early addition of the stabilizer in the polymerization process requires an undesirable choice between production throughput and thermal stability of the polymer. As used herein, “thermal stability” refers to a low rate of acetaldehyde generation, low discoloration, and retention of molecular weight following subsequent heat treatment or other processing.
Late addition of the stabilizer (e.g., after the polymerization process during polymer processing) may provide insufficient opportunity for the stabilizer to fully blend with the polymer. Consequently, the stabilizer may not prevent degradation and discoloration of the polyester. In addition, adding stabilizer during polymer processing is inconvenient and does not provide economies of scale.
U.S. Pat. No. 5,376,702 for a Process and Apparatus for the Direct and Continuous Modification of Polymer Melts discloses dividing a polymer melt stream into an unmodified stream and a branch stream that receives additives. In particular, a side stream takes a portion of the branch stream to an extruder, where additives are introduced. Such techniques, however, are not only complicated, but also costly, requiring a screw extruder and melt piping to process additives. Consequently, such arrangements are inconvenient and even impractical where total additive concentrations are low (e.g., less than one weight percent).
Certain problems associated with late addition of stabilizer are addressed in U.S. Pat. No. 5,898,058 for a Method of Post-Polymerization Stabilization of High Activity Catalysts in Continuous Polyethylene Terephthalate Production, which discloses a method of stabilizing high activity polymerization catalysts in continuous polyethylene terephthalate production. This patent, which is commonly assigned with this application, is hereby incorporated entirely herein by reference.
In particular, U.S. Pat. No. 5,898,058 discloses adding a stabilizer, which preferably contains phosphorous, at or after the end of the polymerization reaction and before polymer processing. This deactivates the polymerization catalyst and increases the throughput of the polyester without adversely affecting the thermal stability of the polyethylene terephthalate polyester. While a noteworthy improvement over conventional techniques, U.S. Pat. No. 5,898,058 teaches adding the stabilizer without a carrier. Consequently, the addition of solids into the polymer necessitates the costly use of an extruder.
Therefore, there is a need for a post-polymerization injection technique that ensures that the late addition of additives during continuous polyethylene terephthalate processes will yield a polyethylene terephthalate polymer whose additives and carriers are integral parts of the polymer resin.